1. Field of the Invention
The present disclosure relates generally to the field of telecommunications and in particular but not exclusively to a method for receiving two decorrelated signals transmitted on a single channel.
2. Description of the Related Art
Transmission band requirements increase with the development of telecommunications.
Generally, the transmission band of a given communication channel is increased by increasing bit loading on a digital data transmission channel according to a given modulation pattern defining a finite set of complex points or information-carrying symbols. In Phase Shift Keying modulation, a set of complex points distributed on the same circle is defined. While the simplest version, known as Binary Phase Shift Keying (BPSK), makes it possible to transmit 1 bit per symbol, QPSK can simultaneously transmit two bits, and 3 bits can be obtained with 8QPSK modulation and 4 bits with 16QPSK modulation.
Quadrature Amplitude Modulation makes it possible to further increase transmission rates by means of a bigger constellation by operating at the same time a phase and shift modulation. A 16QAM or 64QAM modulation makes it possible to increase the transmission rate up to 4 or 6 information bits respectively, especially when associated with an efficient error correcting code system.
However, QAM techniques cannot be implemented when a data transmission channel suffers from latent nonlinearity as occurs in some situations, and in particular in satellite transmissions. As everybody knows, the electronics loaded on to a satellite are subjected to strict constraints of operation, and in particular to minimal power consumption as they are powered by solar cells. Thus, traveling wave tubes that are loaded onto satellites operate in saturated mode in order to minimize electronic current consumption. Nonlinearity is then very important, as illustrated in FIG. 1 where X-coordinates show the amplitude of an input signal of a transponder. On Y-coordinates, the curves respectively represent the output amplitude and its statistical distribution as well as the phase curve of the output signal of the transponder. As can be clearly seen, around the saturation point set to 0 dB, the transponder has a saturation corresponding to a maximum amplitude of the output signal of this transponder.
In the presence of such nonlinearity, it is not possible to resort to constellations and performances of QAM-type constellations. The choice is then confined to simpler modulation patterns, compatible with the nonlinearity affecting the data transmission channel, and in particular to modulation phase shift keying or MPSK. In practice, for satellite transmission, one hardly exceeds the performances of an 8PSK modulation (such as shown in FIG. 2), which corresponds to loading of three information bits.
To increase rate, other alternative techniques should be considered.